On the role of modifier structure in the palladium-catalyzed enantioselective hydrogenation of furan-2-carboxylic acid

Enantio selective hydrogenation of the aromatic ring of furancarboxylic acids is an important new application of cinchona-modified palladium as there is no synthetically useful method yet available for this transformation. Here we report a mechanistic investigation of the hydrogenation of furan-2-carboxylic acid. The 5 wt.% Pd/Al2O3 catalyst was chirally modified by cinchonidine (CD) derivatives, (R)-1-(1-naphthyl)ethylanline derivatives, and (R)-1-(1-naphthyl)-ethanol. Variation of the structure of the modifiers revealed that the major requirement an efficient chiral modifier has to fulfill is the presence of a basic N and an OH function. The relative position of the two functional groups and the acidity (proton donor ability) of the OH group are not critical as indicated by the similar efficiency of 1,2- and 1,3-amino alcohols and amino phenols. The enantioselection is attributed to the fort-nation of a cyclic, 2:1 acid:modifier complex that adsorbs close to parallel to the Pd surface via pi-bonding of the aromatic rings of substrate and modifier. The model can interpret also the effect of a strong acid additive. The poor performance of amine type modifiers is attributed to the formation of too flexible, acyclic structuresShow more